Many power steering apparatuses have been developed in which based on a detected steering torque exerted on a steering wheel by the driver of a vehicle, an electric motor for steering assistance is driven to operate so that the force applied to the steering wheel by the driver is assisted by the rotational force of the motor to provide the driver with good steering feel.
Of the power steering apparatuses of the type described above, there is a so-called column-type power steering apparatus in which a torque sensor and an electric motor are installed on a steering shaft or column, which is operatively coupled to a steering wheel, so as to provide relatively ample space for maintenance out of the elements in the passenger.
In this type of power steering apparatus, the steering column comprises an input shaft operatively coupled to the steering wheel, an output shaft operatively coupled to a motion-conversion mechanism which converts rotational movement of the steering wheel into steering movement of the steerable wheels, and a torsion bar through which the input and output shafts are connected with each other in a coaxial relation. The steering torque exerted on the steering wheel by the driver is detected by the torque sensor such as a potentiometer provided on the junctions of the input and output shafts as a relative displacement between the input and output shafts due to a twist of the torsion bar, e.g., as an output voltage of the potentiometer. The rotational force of the motor is transmitted to an intermediate portion of the output shaft near the location at which the torque sensor is installed.
With the power steering apparatus as constructed above, due to the arrangement where the potentiometer serving as the torque sensor is mounted on the steering column which rotates together with the steering wheel, the application of an input voltage to and the taking out of an output voltage from the potentiometer are effected through slip rings, which are disposed around the potentiometer, and brushes which are secured to a housing surrounding the steering column and which are in electrical and sliding contact with the slip rings. However, such an arrangement involves the following disadvantages: torque detection accuracy is unavoidably reduced due to improper setting in the contact pressure between the brushes and slip rings upon assembly thereof, wear on the brushes over time or the like; as a result of improper electrical contact between the brushes and slip rings caused by vibrations, continuous torque detection becomes impossible.
Further, the above-mentioned conventional power steering apparatus includes a speed-reduction mechanism in the form of a worm gear comprising a worm shaft and a worm wheel, which is interposed between the output shaft and the motor to obtain an appropriate steering speed as well as a sufficient steering assistance in the course of transmission of the rotational force from the motor to the output shaft. In this type of speed-reduction mechanism, however, it is impossible to adjust the engagement clearance or backlash between the worm shaft and the worm wheel upon assembly thereof. As a consequence, variations in the engagement clearance are normally accommodated by enhancing the machining accuracy in the distance between the axis of the worm shaft and that of the worm wheel, i.e., the machining accuracy of a first housing on which the worm shaft is journalled and of a second housing on which the output shaft having the worm wheel fitted thereon is journalled. In this case, however, such high accuracy machining is time-consuming and it is still difficult to eliminate the variations to any substantial extent so that unpleasant transmission noise resulting from excessive engagement clearance is generated, or on the contrary, the transmission resistance at the engaging portions of the worm shaft and the worm wheel is increased due to excessively small amounts of engagement clearance, thus reducing power transmission efficiency. As a result, satisfactory steering assistance can not be obtained.
Moreover, in cases where maintenance and/or repair of the power transmitting portions of the power steering apparatus are conducted after it has been assembled in the above manner, disassembling and re-assembling thereof require a great deal of labor so that maintenance and/or repair is cumbersome and can not be effected in an efficient manner.